1. Field of the Invention:
The present invention relates to a method for the indirect identification of mass flow of a first fluid stream and to an apparatus for carrying out such method, whereby the first fluid stream is mixed with a second fluid stream and the mass flow of the first fluid stream is identified from the difference between the measured mass flows of the second fluid stream and of the mixed stream that has thereby arisen and whereby, for calibration, the mass flows of calibration streams are respectively measured and are employed for correction, as needed.
More particularly, the present relates to a method for the indirect identification of the mass flow of a specimen fluid stream that is mixed with, in particular, a significantly-greater dilution fluid stream to form an total fluid stream that is to be investigated in view of specific constituents of the specimen fluid stream, whereby the mass flow of the specimen fluid stream is identified from the difference between the measured mass flows of the dilution fluid stream and of the total fluid stream, and whereby the mass flows of calibration streams are respectively measured for calibration and are employed for correction, as needed.
Also more specifically, the invention is also directed to an arrangement for the investigation of a specimen fluid stream, comprising a specimen conduit for the specimen fluid stream discharging into a mixing volume, comprising a dilution conduit for a dilution stream likewise discharging into the mixing volume and comprising a mixed conduit for the total fluid stream that leads from the mixing volume to a suction device via an analysis unit, as well as comprising respective mass flow regulators in the dilution conduit and in the mixed conduit following the analysis unit that are connected to an evaluation unit for the indirect identification of the mass flow of the specimen fluid stream, and comprising a calibration arrangement for calibrating the mass flow regulators.
2. Description of the Prior Art:
A controlled mixing or, respectively, dilution of gaseous or liquid fluid streams is frequently required in the control and analysis of processes, as well as generally in measuring and control technology. To this end, flow regulators (for mass or volume streams) are known that offer defined, adjustable fluid streams. For gaseous fluid streams, for example, what are referred to as "thermic mass flow regulators" are known in this context as compact units and are commercially-available devices, these identifying the actual flow with a hot-wire anemometer and offering a certain reference flow on the basis of appropriate follow-up adjustment or readjustment devices. A defined specimen fluid stream G.sub.P can therefore have a generally significantly-different dilution fluid stream G.sub.V added thereto with such regulators, the dilution ratio q therefore resulting on the basis of the expression EQU q=(G.sub.V +G.sub.P)/G.sub.P.
The above inherently, extremely-simple known method can no longer be utilized when the specimen fluid stream is composed of, for example, aggressive substances that would chemically attack the regulators or of contaminated particleladen gases or liquids that continuously contaminate the regulators and mechanically destroy the same over a long term. Only one example in this respect is the dilution of diesel exhaust gas with air. Such a dilution is currently required by most jurisdictions before the measurement of contents of harmful substances, particularly of the particle content, in the exhaust gas in order to simulate emission conditions.
The above known method was developed for the indirect identification of the mass flow of the specimen fluid stream that is therewith necessary, the dilution fluid stream G.sub.V and the mixed stream or, respectively, total fluid stream G.sub.T thereby resulting being regulated therewith and the specimen fluid stream or, respectively, the dilution being calculated therefrom in accordance with the relationship EQU G.sub.P =G.sub.TOT -G.sub.V or, respectively, q=G.sub.TOT /(G.sub.TOT -G.sub.V).
The identification or, respectively, follow-up readjustment of the mass flow is therefore limited, on the one hand, to the unloaded dilution fluid stream and, on the other hand, to the mixed stream or, respectively, total fluid stream (from which the loading by the specimen fluid stream can, in turn, already be filtered out at this location), this avoiding the above-described disadvantages.
This latter, so-called difference control method can be extremely-well utilized given relatively-low mixing or, respectively, dilution ratios (q&lt;10). The throughput or, respectively, flow regulators for the total fluid stream G.sub.tot and for the dilution fluid stream G.sub.V must, in fact, generally be calibrated from time-to-time since, in particular, the measured value pick-ups of the regulating units are subject to chronological drifts and fluctuations; since, however, the accuracy and reproducibility of these units lies about or, respectively, below 1% based on the current technical standard, the specimen fluid stream G.sub.P can usually still be set with adequate accuracy for low dilution rates.
However, there is currently frequently the demand to also be able to set higher dilution ratios, whereby, however, it has been shown that extremely-great inaccuracies then occur in the setting of the specimen fluid stream G.sub.P and of the dilution ratio q, even when the flow regulators for the total fluid stream G.sub.tot and the dilution fluid stream G.sub.V are calibrated with the same units. This becomes clear on the basis of a simple numeric example. When, for example, the regulator for the total stream G.sub.TOT and for the dilution fluid stream G.sub.V is calibrated 0.5% too low and the regulator for the dilution fluid stream G.sub.V is calibrated 0.5% too high, then, given the setting of a dilution ratio of q=20, one obtains an actual dilution ratio of q'=24.72 in accordance with the above equation, i.e. an error of 23.6%.
In order to suppress this systematic error, a feature of the difference regulating method has been disclosed in accordance wherewith small sub-streams are branched off from the specimen fluid stream and from the total fluid stream, the concentration of a characteristic constituent then being measured at the sub-streams and being employed for the readjustment of the flow regulator for the dilution fluid stream G.sub.V and/or the total fluid stream G.sub.TOT. For the above-addressed example of the dilution of diesel exhaust gas, for example, the measurement of exhaust gas constituents CO.sub.2 or NO.sub.X is available. This feature, however, requires a significantly-greater expense that, in the view of costs and apparatus equipment, frequently amounts to a multiple of that for the actual mixing or, respectively, dilution device itself.